Abstract: A set of three or more acoustic transponders positioned below the surface of the ocean is provided with logic circuitry that enables location of transponder positions merely by listening to transponder signals and also enables location of an unknown receiving station merely by listening to transponder signals. The transponders are self-synchronizing to eliminate the need for precision timing over long periods. In a first mode, each transponder transmits a signal to the surface and retransmits upon receipt of its echo to thereby provide depth data. In a second mode the transponders transmit to each other and retransmit an acknowledge signal upon receipt of a reply from one of the other transponders. This provides data to enable a remote station to calculate distances between the respective transponders.